Just install, set empty default variable for ldap server.

defaults/main.yml

 ---
 # defaults file for ansible-freeradius
+
+freeradius_ldap_server:

tasks/main.yml

 ---
 # tasks file for ansible-freeradius
+
+- name: apt install
+  with_items:
+  - freeradius
+  - freeradius-ldap
+  apt:
+    name: '{{ item }}'
+    state: present
+    update_cache: yes
+    cache_valid_time: 3600